Transistor current sources are widely used in integrated circuits as biasing elements and as load devices :for amplifier stages. The purpose of a current source in biasing is to provide a source of current that is insensitive to power supply variations and to changes in temperature. Such current sources may be implemented in bipolar transistor processes as well as in metal oxide semiconductor processes.
A typical bipolar current source is shown in FIG. 1. A pair of npn transistors are configured in a typical current source arrangement. The bases of the transistors Q1, Q2 are tied together and the transistor Q1 has its collector coupled to its base. A positive power supply V.sub.cc provides a voltage to the transistors Q1 via a resistor R. The current I.sub.c2 that flows in the collector of transistor Q2 will be approximately equal to the current I.sub.c1 in transistor Q1. The circuit of FIG. 1 is discussed in more detail in "Analysis and Design of Analog Integrated Circuits," 2nd Edition, Paul R. Gray and Robert G. Meyer, 1977, pages 233-237.
FIG. 2 shows an improvement over the current source of FIG. 1. In FIG. 2, an npn transistor source consisting of transistors Q1, Q2 is set at a predetermined voltage bias level. It is set at that predetermined voltage bias level by transistors Q3 and Q4. Transistors Q3 and Q4 are base-collector crosscoupled. Then, the current flowing through a diode Q25 is mirrored to provide a current sink for npn transistors. The current sink is provided by current I.sub.n that flows through transistor Q29. Transistor Q29 mirrors the current I.sub.n through diode Q25 via transistor Q26 and diode Q27. Current is separately supplied to the pnp transistors through transistor Q28. Thus, the circuit in FIG. 2 provides a current source I.sub.p for the pnp transistors and a current sink I.sub.n for npn transistors.
However, the circuit of FIG. 2 is not symmetrical and does include current mirroring twice. For this reason, the currents I.sub.p and I.sub.n are not as precisely symmetrical as desired in precision amplifiers. As a result of the mirroring, an Early voltage error is introduced and that error together with beta errors of the transistors requires additional transistors to modify the current source and achieve symmetrical current sources for npn and pnp transistors.